well it is true that 15 inch speakers are slow, with 2 15's you would have 30 inches of speaker and would be somthing to brage about and with 2 12's you would have 24 inches so of coars the 15's would be loader BUT only with certan songs the 12's would be great for ANY kind of music you want to listen to but if you want to brage about them i would get 3 12 inch speakers then you would have 36 inches speakers that would kick alot of azz and would look realy cool (in my opinion)
Hip Hop is a little slower than you think. My 15" is waaaay louder when it's playing Nelly than it is when it's playing Metallica. Hip Hop is mainly sustained low bass that can be fast at times. Fast bass is usually in rock songs and is closer to 120hz rather than 80hz so 10"s and 12"s are better suited for rock. What I'm gonna do is add 2 10"s to my system and maybe another 15" or 12" depending on space.
bryan, you dont measure speakers with sum of diameter, measure by surface area...and with circles (round subs) area = (pi) radius squared
A of 15" = ~176 square inches A of 12" = ~113 square inches A of 10" = ~78 square inches
a 15 inch sub will give you much slower response in a ported box than a sealed box if you're saying u wanna open ur trunk and show off two 15" subs, get audiobahn crap..looks good but sucks
if you want good sound, check out an adire audio tempest (15" sub), not the greatest looking sub but will hit 103+ dB in a sealed box and have excellent SQ and it's only $150 and only needs 400-750w rms to run
you can also check out a pair of shivas (also adire audio) $125 each and they arent for looks either, but for sound again
cerwin vega has some good subs and they look pretty sweet too...it gives you some from both worlds
I have 2 15's (alpine r15's)... 2 12's (alpine r12's)...im also about to get 2 10's (alpine r10's)...i have 4 amps 1200 on each sub...It is GREAT...but i have also fit this all in a Escalade...i have 3 tv's...new speakers in doors...it has all costed about 8k and i got 24's coming soon....soooo if any1 want to chalange i live in FL...